The present invention relates to a process for measuring the time unit (ETU=Elementary Time Unit) during a dialogue between a reader and a smart card, so as to configure the reading device.
The state of the art at present proposes a technique of sampling of the first octet or character, then a calibration by means of software.
During the start-up of a dialogue between a reader and a smart card, the reading device must be configured to sample the data transmitted in series or in an asynchronous fashion by the card (width of each transmitted bit=1 ETU).
To the nearest decimal point, the width of each bit is ETU=372 CLK, namely 1/9600 second for a card timed to 3.57 megahertz (MHz). However, certain cards have a value to the nearest decimal point of 512 CLK.
There is known from EP-A-0 347 894 a card with integrated circuits having recognition means (22) to determine whether the charged card (23) has an identifiable frequency.
However, only a limited number of frequencies can be identified according to this technique.
There is also known from the Patent Abstract of Japan JP 04-319756, an interface of a communication series adapted to reconstitute the frequency of transmission in the connection series.
According to the invention, a processor for measuring the time unit during start-up of the dialogue permits operating in a transparent manner with cards having ETUs that are different to the nearest decimal point.
The old techniques of sampling require considerable time and use for that purpose a microprocessor. The invention permits increasing the speed of taking account of the time unit by not using software, but rather conventional computer hardware.
To this end, the present invention relates to a process for measuring the time unit (ETU=Elementary Time Unit) during a dialogue between a reader and a smart card, so as to configure the reading device, characterized in that it consists:
in taking account of the descending front of the start bit,
taking account of the ascending front of said start bit, and
measuring the time between said descending and ascending fronts,
which permits the automatic calibration of the communication processor.
The measured time corresponds to the low level of the start bit.
The time is measured as a number of cycles of the card clock.
The automatic calibration consists:
in defining the value of a half unit of time,
carrying over this value after the ascending front of the start bit to define the middle of the following bit,
carrying over successively the value of a time unit to define the position of each successive bit.
The processor can be used with different values of the time unit, such as ETU=372 CLK or ETU=512 CLK.
This process, in which the first character is constituted by the start bit (low level during a unit of time) then a so-called time sharing bit or TS (high level during the unit of time) is characterized in that after the TS bit, it is possible to reprogram or to adjust the value of the unit of time measured.
The processor permits the elimination of parasitic signals, which is to say values of the time unit that are too small.